DESCRIPTION: The overall goals of this research program are to determine how simple vertebrate auditory systems function to give rise to the sense of hearing defined in a comparative and evolutionary context. We argue that the sense of hearing functions in species of all vertebrate classes primarily in the detection, identification, classification, and the location of both animate and inanimate sound sources. In addition, we hypothesize that the neural mechanisms underlying sound source determination consist of primitive components and strategies that are widely shared among vertebrate species, including human beings. This view motivates and guides the proposed experiments.Preliminary work has demonstrated that in spite of wide differences in the inner ears of fishes compared with tetrapods, the sense of hearing revealed in psychophysical and neurophysiological studies on goldfish appears to be indistinguishable in fundamental aspects from that of terrestrial animals, including mammals. The present proposed behavioral, neurophysiological, and neuroanatomical experiments are designed to help evaluate the hypothesis that the sense of hearing and its underlying neural mechanisms are widely shared among the most numerous and successful vertebrates, the fishes. Comparisons of these results with those for goldfish and tetrapods will help reveal the dimensions on which variation occurs among vertebrate ears. Two fundamental aspects of sound source determination will be studied: sound source localization and sound source identification and classification. These studies will be carried out at three levels of analysis: behavior (psychophysics and scaling type of studies), single-cell neurophysiology at various levels of the auditory system, and comparative neuroanatomy. The goals are to compare fishes with humans and other tetrapods with respect to the neural mechanisms underlying sound source determination. These results will help in the development and evaluation of animal models for human hearing and help establish the comparative and evolutionary contexts within which the human sense of hearing can be better understood.